Open AccessMagnetic anisotropy behaviour of pyrrhotite as determined by low‐ and high‐field experiments
Author(s) -
MartínHernández F.,
Dekkers M. J.,
BominaarSilkens I. M. A.,
Maan J. C.
Publication year - 2008
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2008.03793.x
Subject(s) - pyrrhotite , anisotropy , condensed matter physics , remanence , grain size , magnetization , magnetic susceptibility , magnetic anisotropy , rock magnetism , geology , magnetic hysteresis , materials science , mineralogy , magnetic field , physics , optics , composite material , pyrite , quantum mechanics
SUMMARY Here we report on the sources of magnetic anisotropy in pyrrhotite, an iron sulphide present in many rocks as an important carrier of the Natural Remanent Magnetization. While the magnetic hysteresis parameters of pyrrhotite are well known, the existing database concerning its anisotropy behaviour is patchy and ambiguous. Therefore, a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized. Before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring Curie temperatures, hysteresis loops, IRM acquisition curves, and FORC diagrams (the latter three all at room temperature). First the variation of the low‐field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 A m −1 . Existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence. This enables estimating the grain size for unimodal pyrrhotite distributions in rocks. Measured trends are better fitted with an exponential function than with a Rayleigh Law style function. Based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples (of the original 11) were selected for the high‐field anisotropy determinations within the basal plane. Those data were acquired with a torque cantilever‐type magnetometer. As expected, most single crystals showed a pure 6–θ curve within their basal plane because of the easy axis configuration. In some crystals, however, lower harmonic terms overlapped the 6–θ term. This may be the dominant source of the observed variation in magnetic anisotropy properties. Torque data of three of the six samples were of sufficient quality to allow evaluation of K 1 . Re‐evaluation of existing torque data and including the present newly derived determinations, yields for the anisotropy constant of pyrrhotite within the basal plane K 1 : (2.7 ± 0.2) 10 4 Jm −3 . This is over an order of magnitude more precise than the sparse existing K 1 data; only the value reported by Mikami and co‐authors in 1959 agrees with the new determination. With this firmly established K 1 value meaningful anisotropy models are now possible for pyrrhotite‐bearing rocks.